In general, there are two different types of EDM apparatus for machining a workplace in a work tank using electrical discharges; i.e., die sinking EDM apparatus and wire-cut EDM apparatus.
In a die sinking EDM apparatus, a cavity of a desired configuration is formed in the workplace. A dielectric fluid is introduced into the work tank in order to submerge the workplace which is fixed on a work table in the work tank. During machining operations, a tool electrode having a three-dimensional shape advances toward the workplace, usually from above along the Z axis. A door is typically provided on the front wall of the work tank to provide access to the workplace and the tool electrode. However, the work tank walls still impair accessibility to the workplace and electrode. Further, considerable front clearance space is required to open and close the door which swings out horizontally. In addition, in cases where an automatic pallet changer is used to carry out unmanned machine operations over a period of several days, the tank walls become an obstacle to changing the workplace.
U.S. Pat. No. 4,712,288 to Lodetti, et al. and Japanese Laid-Open Patent Publication No. 61-270023 disclose a die sinking EDM apparatus having the work tank in which a rear wall is fixed and the other three walls are of a U-shaped, unitary construction which can be lowered to provide access to the work table.
Also, Japanese Laid-Open Patent Publication No. 2-48113 discloses a die sinking EDM apparatus having a work tank consisting of a base container member and at least one movable frame member which can move vertically. The movable frame member moves down into the base container member so that the four walls of the work tank can telescope as a whole in a sealing relationship.
In a wire-out EDM apparatus, the workpiece is usually fixed on a work stand or work stands in the work tank which is movable in a horizontal plane. A lower arm extends into the work tank through an opening in one of the walls thereof and an upper arm is positioned above the lower arm. An electrode wire travels between an upper wire guide block and a lower wire guide block which are mounted on the ends of the upper arm and the lower arm, respectively. The electrical discharges occur between the traveling wire and the workpiece through the dielectric fluid while the relative movement between the position of the wire electrode and work tank (and hence the workpiece) in the X and Y axes is controlled since in a wire-cut EDM apparatus, water or a water solution may be used as a dielectric fluid, so-called "submerged machining" in which the workpiece is submerged in the work tank during machining is generally desirable, for example, to prevent rust.
As in a die sinking EDM, the tank walls of a wire-cut EDM apparatus impair access by the user when loading the workpiece onto the work stand and when vertically aligning the electrode wire in preparations for machining. Japanese Laid-Open Patent Publication No. 2-298435 discloses a wire-cut EDM apparatus in which three tank walls, comprising the front and both side walls, include a bellows which moves up and down as it is expanded and contracted. However, because of the bellows' construction this apparatus is not designed for submerged machining. If the work tank is filled with a dielectric fluid, the walls of the bellows will expand, as such walls are of a relatively flimsy construction.